The field of the invention is sensors for protecting alternating current electrical equipment against damage from ground faults.
Numerous ground fault sensors are known in the art and include a current transformer that couples to the power lines leading to the electrical equipment to be protected. If arcing to ground, or a short circuit to ground should occur in one or more of the power lines, the current flowing to the electrical equipment will differ from that leaving the electrical equipment and a net current flow in one direction will be established. This ground fault current induces a voltage in the transformer secondary winding and circuitry is connected thereto which shuts down the electrical equipment when the fault current reaches a preset level. When applied to motor starters, for example, the circuit is connected to deenergize the line contactor when the ground fault current reaches a preset magnitude.
Due to the limitations in the current interrupting capacity of the line contactor in a motor starter, it is more desirable to allow the fusing elements in the motor starter to interrupt very high ground fault currents. To accomplish this, in prior circuits a substantial time delay is built into the ground fault detection circuitry to allow time for the fusing element to operate when the ground fault current is of such magnitude. This time delay in the response of the ground fault detection circuit is undesirable in cases where the ground faults are less catastrophic.